When developing radio equipment, the aim is to achieve the maximum performance and the optimum functional characteristics in as small a space as possible.
The physical dimensions of passive circuits such as filters are determined by the design frequency. Consequently, there is only a small degree of freedom from the viewpoint of the flexible packaging of the respective components.
For example, in a bandpass filter which includes an E-plane parallel metal plate, the level of satisfaction with the standard in the pass bands (i.e., the abruptness of boundaries with stop bands) is decided solely by the number of stages. Because of this, there are cases when the overall filter length is too long for the space provided.
In bandpass filters in which a plurality of resonators are aligned in a row, technology is known in which, by coupling resonators together by crossing one or more resonators, an attenuation pole is created on the outside of the pass band thereby improving the attenuation characteristics.
“The combline bandpass filter” disclosed in Patent document 1 is known as a bandpass filter in which cross-coupling lines are formed. In the invention disclosed in Patent document 1, on the inside of one side surface of a metal case, a metal coupling loop is installed with one end thereof being bent in the direction of the open end of a resonance conductor, and the other end thereof being bent in the direction of the short-circuit end of the resonance conductor.